Homologues of the human HLA-A and -B MHC class I loci have been found in great apes and Old World primates suggesting that these two loci have existed for at least 30 million years. The C locus, however, shows some sequence similarity to the B locus and has been found only in gorillas, chimpanzees and humans. To determine the age of the MHC class I C locus and to examine the evolution of the A and B loci we have cloned, sequenced and in vitro translated sixteen MHC class I cDNAs from two unrelated rhesus monkeys (Macaca mulatta) using both cDNA library screening and PCR amplification. Analyses of these sequences suggest that the C locus is not present in the rhesus monkey, indicating that this locus may be of recent origin in gorillas, chimpanzees and humans. The rhesus monkey's complement of MHC class I genes includes at least one expressed A locus and at least two expressed B loci, indicating that a duplication of the B locus has taken place in the lineage leading to these Old World primates. Interestingly, several poorly expressed A and B locus cDNAs were isolated from these two rhesus monkeys. Comparison of rhesus monkey MHC class I cDNAs to their primate counterparts reveals fundamental differences between MHC class I and class II evolution in primates. While MHC class II allelic lineages are shared between humans and Old World primates, no such trans-species sharing of allelic lineages is seen at the MHC class I loci. Key Words SIV, CTL, Rhesus monkey, MHC